A series of shootings on the home turf of two of the city's largest Latino gangs has brought violence west of Downtown back to record-breaking levels.

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Gang-related killings have soared in the Los Angeles Police Department's Rampart Division--with 21 murders in August--after two years of decline, according to law enforcement officials.

Rampart, which includes the Pico-Union and MacArthur Park areas, is the home of the 18th Street and Mara Salvatrucha gangs, and has more homicides than any other LAPD division. Its gang activity often foretells trouble throughout the city, officials say.

"For the last few years Rampart's been a weather vane; when it gets busy here, it gets busy everywhere else," said Rampart Detective Terry Wessel, a gang specialist.

The latest round of gang killings in Rampart may reveal a breakdown of the Mexican Mafia's control over heroin sales in the area, according to a law enforcement official who monitors the prison gang. A much-vaunted Latino gang truce in the San Fernando Valley is believed to be breaking down for the same reason.

Two years ago the Mexican Mafia, which consists of incarcerated members of Latino street gangs, ordered Los Angeles' Latino street gangs to stop drive-by shootings. The Mexican Mafia was concerned that the shootings had increased police attention to gangs, hurting street drug sales, according to police memos.

Known as La Eme, Spanish for the letter M, the Mexican Mafia had taxed drug dealers and assigned zones for heroin sales to different gangs in the Rampart area, according to a law enforcement source. With the recent death or imprisonment of several of La Eme's enforcers who had kept order in the area, street gangs began to fight for turf.

With 99 homicides through September, Rampart has already passed last year's total of 98, and is at the record-breaking pace of 1992, when the division had 149 homicides.

Most of the victims in the latest round of gang homicides have been members of the 18th Street gang, believed to be the largest in California, with dozens of "cliques," or informal chapters, spread throughout California. In addition to the mostly Chicano 18th Street and the Salvadoran Mara Salvatrucha, 13 other gangs are active in the Rampart area, Wessel said.

Law enforcement officials who monitor gangs in the central city are split over the causes of the increased violence. Gang detective Wessel rejects the theory that La Eme controlledstreet gangs, and that the recent violence reflects a breakdown in their authority.